The use of zirconium as a cladding in nuclear fuel rods is known. There are, however, five common isotopes of zirconium: zirconium 90, zirconium 91, zirconium 92, zirconium 94, and zirconium 96, which isotopes have very different capture cross-sections for thermal neutrons. They are: 0.03 b (barns) for zirconium 90, 1.1 b for zirconium 91, 0.2 b for zirconium 92, 0.055 b for zirconium 94, and 0.02 b for zirconium 96. A significant improvement would be provided in nuclear fuel cladding if the zirconium 90 isotope concentration could be increased, in zirconium used in the cladding, above the about 51.5 percent level found in natural zirconium. The higher the level of zirconium 90 in the cladding, the more beneficial the properties.
There are several known isotope separation techniques, such as gas diffusion, gas centrifuging, electromagnetic separation, and chemical exchange, for the separation of lithium, boron, calcium, titanium, and uranium isotopes, and some proposals to separate zirconium isotopes, such as by use of a laser or photoexcitation and separation steps. Isotope separation processes using lasers incur considerable expense.
In the separation of zirconium from ores that normally contain hafnium values, the use of solvent extraction of thiocyanate complexes of these metals is generally used. This process comprises extracting an aqueous solution of hafnium and zirconium salts containing ammonium thiocyanate with an ether, or methylisobutyl ketone (MIBK), solution of thiocyanic acid, with the hafnium preferentially passing to the organic layer, while the zirconium remains in the aqueous layer. Such a process is referred to in U.S. Pat. No. 3,069,232, which teaches an improvement where a saturated solution of ammonium sulfate is used, instead of sulfuric acid, to re-extract hafnium values from the organic phase.
We have now discovered that the complexes such as thiocyanate complexes, of zirconium isotopes, such as a zirconium 90 isotope component or a zirconium 91-96 isotope component, of a mixture of zirconium 90-96 isotopes, in an aqueous phase can be preferentially extracted to an organic phase so as to enrich a zirconium isotope component content in the aqueous phase. Exchange can then be effected to produce an enriched zirconium isotope component product.
It is an object of the present invention to provide a process for the enrichment of either a zirconium 90 isotope component or a zirconium 91-96 isotope component in a mixture containing those two components.
It is another object of the present invention to provide a solvent extraction-exchange process for the enrichment of either the zirconium 90 component or the zirconium 91-96 component in a mixture containing those two components.
It is a further object of the present invention to provide a solvent extraction-exchange process for enriching either the zirconium 90 isotope component or the zirconium 91-96 isotope component of an aqueous zirconium-containing feed stream, from a hafnium-zirconium separation system using a thiocyanate complex.